She was 18 at the time, and didn't imagine that clicking "accept" would start her on a path to four years of prostitution across the country. "Nina" is a pseudonym; CNNMoney agreed to change the names of the victims in this article to protect their privacy.

Upper middle-class and college-bound, Nina had her plans derailed in her senior year of high school after her mother was sentenced to two years in prison for financial crimes. Lonely and looking online for male attention, she started messaging back and forth with a man who said he was falling for her. They talked about trips they'd take together as a couple, and about marriage, maybe kids.

"He sold me the biggest dream in the world," she says. "I thought he really did like me and we were going to live this fairy-tale life together."

They exchanged online messages for about a month. That September, while Nina's friends went off to college, she traveled the two and half hours from home to meet her Facebook beau in person.

The fairy tale ended fast. Almost immediately after she arrived in Seattle, he dropped her off on a street where prostitutes troll for customers and told her she was going to "catch dates."

Many would have run, but Nina says her deteriorating family life left her with a sense of desperation. She was smitten, and willing to do anything for the man she thought loved her. So she stayed.

Keeping the attention of her "boyfriend" required selling herself for sex, Nina learned. He was a pimp -- and she was one of a growing number of women recruited on social networks for sex trafficking.

There are no hard statistics on the scope of the problem. Law enforcement officials don't track how sex workers are recruited into the field, and unless the victims are underage, prostitution is typically a low-priority crime.

But recent prosecutions in California, Virginia and Washington, along with interviews CNNMoney conducted with victims and those investigating these crimes, illustrate how social networks are helping traffickers lure in victims like Nina.

"Pimps are professional exploiters," says Andrea Powell, executive director of Fair Girls, an organization that helps victims of sex trafficking. "Often they're just spamming a whole bunch of girls with messages like, 'Hey, you look cute. I could be your boyfriend.'"

That's one way Justin Strom -- aka "J-Dirt" -- recruited the high-school girls he and his followers trafficked in Alexandria, Va., an affluent suburb on the outskirts of Washington, D.C. For six years, the members of Strom's "Underground Gangster Crips" gang operated a prostitution ring that ensnared at least eight 16- and 17-year olds, according to court documents.

The girls were rented out to five to 10 customers each on a typical night. The going rate was around $30 for 15 minutes of sex.

Social networks were among Strom's preferred hunting grounds.

The group "searched Facebook for attractive young girls, and sent them messages telling them that they were pretty and asking if they would like to make some money," one witness told a Federal Bureau of Investigation agent investigating the case. The court records include a trail of those messages.

Strom had a collection of fake Facebook accounts. On one of them, for "Rain Smith" investigators found more than 800 messages sent out to potential targets.